In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, an image is generally formed on a surface of a sheet such as a paper sheet or a plastic film through the following process steps.
First, a surface of a photoreceptor body having photoelectric conductivity is evenly electrically charged and, in this state, exposed to light, whereby an electrostatic latent image corresponding to an image to be formed on the sheet is formed on the surface of the photoreceptor body (charging step and exposing step).
Then, toner (minute color particles) preliminarily electrically charged at a predetermined potential is brought into contact with the surface of the photoreceptor body. Thus, the toner selectively adheres to the surface of the photoreceptor body according to the potential pattern of the electrostatic latent image, whereby the electrostatic latent image is developed into a toner image (developing step).
Subsequently, the toner image is transferred onto the surface of the sheet (transfer step), and fixed to the surface of the sheet (fixing step). Thus, the image is formed on the surface of the sheet.
In the developing step out of these process steps, a toner supply roller made of a rubber foam having a predetermined roller resistance is used for supplying the toner to a surface of a toner carrier such as a developing roller for developing the electrostatic latent image formed on the surface of the photoreceptor body into the toner image.
The toner supply roller is required to have the lowest possible hardness so as not to break toner particles held between the toner carrier and the toner supply roller, and to have the most uniform and greatest possible foam cell diameters so as to transport a sufficient amount of the toner to the toner carrier by a single transport operation.
In order to meet the requirements, JP-4067893-B proposes that a rubber component, a crosslinking component for crosslinking the rubber component and a foaming component for foaming the rubber component are blended together to prepare a rubber composition, and the rubber composition is extruded into a tubular body which is in turn foamed and crosslinked in a vulcanization can by pressure and heat, whereby the toner supply roller is produced as having a predetermined expansion ratio and a predetermined cell diameter distribution.
An ion-conductive epichlorohydrin rubber, and at least one rubber selected from the group consisting of an acrylonitrile butadiene rubber (NBR), a chloroprene rubber (CR) and an ethylene propylene diene rubber (EPDM) are used in combination as the rubber component.